As air travel has increased over the past decades, airport facilities have become more crowded and congested. Minimizing the time between the arrival of an aircraft and its departure to maintain an airline's flight schedule, and also to make a gate or parking location available without delay to an incoming aircraft, has become an airline priority. The safe and efficient ground movement of a large number of aircraft simultaneously into and out of the ramp and gate areas has become increasingly important. As airline fuel costs and safety concerns and regulations have increased, use of the aircraft main engines is no longer the best option for achieving the desired safe and efficient ground movement.
Various alternatives to the use of an aircraft's main engines to move an aircraft on the ground have been tried. The use of a tug or tow vehicle to move an aircraft into and out of a gate or parking location can eliminate the need to use the aircraft main engines. This option, however, is not without its own challenges and costs. More ground vehicles, requiring more fuel and more ground personnel to operate them, add to an already congested environment in the gate area. Restricted use of the aircraft engines on low power during arrival at or departure from a gate is an additional option. This option is also problematic, however. Not only are the engines designed to operate at high speeds at altitude and not to drive the aircraft on the ground at under 30 miles per hour, they consume very large amounts of fuel on the ground. The engines are also noisy, and the associated safety hazards of jet blast and engine ingestion in a congested area are significant concerns that cannot be overlooked.
The use of a motor structure integrally mounted with a wheel to rotate the wheel and drive an aircraft has been proposed. The use of such a structure, ideally, should move an aircraft with no or, at most minimal, use of an aircraft's main engines or the external assistance of tow vehicles. In U.S. Pat. No. 2,430,163, for example, Dever describes a motor that may be incorporated in an aircraft landing gear wheel, but does not suggest integrating any type of gearing or similar drive structures with this wheel motor. U.S. Pat. No. 3,977,631 to Jenny describes a motor assembly selectively coupled to an aircraft wheel that includes a conventional reduction gear assembly, but there is no suggestion that any other kind of motor drive structure could be employed. U.S. Pat. No. 7,445,178 to McCoskey et al and U.S. Pat. No. 7,226,018 to Sullivan describe, respectively, a powered nose aircraft wheel system with a multifunctional wheel motor driven by a planetary gear assembly or a direct drive and an aircraft wheel hub motor/generator with a stack of alternating rotor and stator disks, in which the rotors are coupled to the wheel. None of the aforementioned patents suggests motor drive or gearing systems useful for producing aircraft self-propelled ground movement other than those disclosed, which are the traditional gear arrangements conventionally used to drive motors.
Conventional gearing, such as the planetary and other gear systems described the patents discussed above, is only one possible option for driving an electric motor. Other drive systems include, for example, strain wave gearing, known as harmonic drive. Harmonic drive is a special type of mechanical gear system based on elastic dynamics utilizing the flexibility of metal that produces improvements over traditional gear arrangements, including helical and planetary gearing. A type of harmonic gearing is shown and discussed in UK Patent Application Publication No. GB 2453027, owned by the assignee of the present invention. The application of the harmonic gearing arrangement described therein is not discussed, and there is no suggestion that it could be used to drive motor components in an aircraft drive wheel.
U.S. Pat. No. 7,469,858 to Edelson, owned in common with the present invention, describes a gear system for an aircraft wheel motor that provides the necessary torque with reasonable system mass and a mechanism for automatically decoupling the high gear ratio needed to drive the load from the load if the load overhauls. While the aforementioned gearing system is described to be located in or near a drive wheel, it is not suggested that the gearing could be replaced by harmonic drive or any other type of gearing
U.S. Patent Application Publication No. US2009/0294578 to Humphrey describes the use of harmonic drive in an aircraft landing gear steering system, in which a turning member is coupled to a landing gear leg through a harmonic drive mechanism. Humphrey is completely silent, however, with respect to whether the aircraft landing gear wheels are powered or otherwise driven by a drive motor or any other power source and does not mention the application of harmonic drive to drive aircraft wheels.
None of the foregoing art suggests a drive system capable of powering an aircraft drive wheel during ground travel that includes harmonic drive. This art, moreover, does not contemplate a wheel drive system with harmonic drive that can be installed or retrofitted in existing aircraft without the modification of other landing gear structures.
A need exists, therefore, for an aircraft wheel drive system including a drive motor with harmonic drive capable of generating optimum torque for driving any aircraft wheel and, thus, the aircraft during ground travel. A need further exists for a harmonic drive system for any aircraft drive wheel specifically configured to drive a motor mounted on the aircraft drive wheel to move the aircraft on the ground independently of aircraft engines or tow vehicles.